1. Field of the Invention
The present invention relates to balanced acoustic wave filter devices that use acoustic waves, such as surface acoustic waves or boundary acoustic waves. More specifically, the present invention relates to a balanced acoustic wave filter device having a balanced-to-unbalanced conversion function using a plurality of acoustic wave filter elements defined by longitudinally coupled resonators.
2. Description of the Related Art
Surface acoustic wave filters are often connected as band pass filters between antennas and differential amplifiers in mobile communication devices. In such a case, an antenna inputs and outputs unbalanced signals. On the other hand, a differential amplifier inputs and outputs balanced signals. Thus, a component having a function of unbalanced-to-balanced conversion must be disposed between the antenna and the differential amplifier. Thus, when a surface acoustic wave filter having a balanced-to-unbalanced conversion function is used as a band pass filter, the component for implementing a balanced-to-unbalanced conversion function, i.e., a balun, may be omitted. In view of this advantage, various types of balanced surface acoustic wave filters having a balanced-to-unbalanced conversion function have been proposed.
However, the characteristic impedance of an antenna is usually about 50Ω, while the characteristic impedance of a differential amplifier is usually about 100Ω or higher, and is occasionally about 1,000Ω. Thus, a surface acoustic wave filter having a balanced-to-unbalanced conversion function must also have an impedance conversion function.
Japanese Unexamined Patent Application Publication No. 2002-290203 discloses a surface acoustic wave filter device having a balanced-to-unbalanced conversion function together with an impedance conversion function.
FIG. 10 is schematic plan view showing an electrode structure of a balanced surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203. A surface acoustic wave filter device 501 has an unbalanced terminal 502 and first and second balanced terminals 503 and 504. The unbalanced terminal 502 is connected to first and second surface acoustic wave filters 505 and 506, each defined by longitudinally coupled resonators. The surface acoustic wave filter 505 includes first to third interdigital transducers (IDTs) 505a to 505c, and the surface acoustic wave filter 506 includes first to third IDTs 506a to 506c. That is, each of the surface acoustic wave filters 505 and 506 is a surface acoustic wave filter including three IDTs defined by longitudinally coupled resonators. On either side of the IDTs 505a to 505c in a direction of propagation of surface acoustic waves, reflectors 505d and 505e are provided. Similarly, on either side of the IDTs 506a to 506c in the direction of propagation of surface acoustic waves, reflectors 506d and 506e are provided.
The second IDTs 505b and 506b in the middle are commonly connected to the unbalanced terminal 502. The IDTs 505a and 505c on either side of the surface acoustic wave filter 505 are commonly connected to the first balanced terminal 503. The first and third IDTs 506a and 506c on either side of the surface acoustic wave filter 506 are commonly connected to the second balanced terminal 504. The phases of the first and second surface acoustic wave filters 505 and 506 differ by 180 degrees.
In the surface acoustic wave filter device 501 disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203, the direction of propagation of acoustic waves is the same in the first and second surface acoustic wave filters 505 and 506. Furthermore, a gap d1 between the first and second surface acoustic wave filters 505 and 506 and a thickness t of a piezoelectric substrate satisfy d1≦2.3×t or d1≧2.8×t. The disclosure of Japanese Unexamined Patent Application Publication No. 2002-290203 describes that these features improve transmission characteristics.
Furthermore, in the surface acoustic wave filter device 501, the input impedance of each of the surface acoustic wave filters 505 and 506 is denoted by Z, and the impedance at the unbalanced terminal 502 is approximately Z/2 since the impedance corresponds to an impedance of a parallel connection of the input impedances of the surface acoustic wave filters 505 and 506. The impedance at each of the balanced terminals 503 and 504 is approximately 2Z since the impedance corresponds to an impedance of a series connection of the impedances of the surface acoustic wave filters 505 and 506. Thus, the ratio of the impedance at the unbalanced terminal 502 to the impedance at the balanced terminals 503 and 504 can be chosen to be approximately 1:4.
As described above, in the surface acoustic wave filter device 501, the ratio of the impedance on the side of the unbalanced terminal 502 to the impedance on the side of the balanced terminals 503 and 504 is chosen to be approximately 1:4.
Thus, for example, the surface acoustic wave filter device 501 described above can be used suitably when the input/output impedance of an antenna is approximately 50Ω and the characteristic impedance of a differential amplifier connected at a subsequent stage of the surface acoustic wave filter device is approximately 200Ω.
Recently, however, the characteristic impedance of a differential amplifier used in such an application is typically greater than about 200Ω, and is occasionally about 1,000Ω.
Thus, even when the surface acoustic wave filter device 501 disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203 is used, when a differential amplifier having such a large impedance is connected to a subsequent stage thereof, an additional impedance conversion component must be provided. That is, the surface acoustic wave filter device 501 disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203 does not provide a sufficient a sufficient impedance conversion function. Thus, a demand exists for a balanced surface acoustic wave filter device in which the input/output impedance ratio can be further increased.
Also recently, in addition to surface acoustic waves, acoustic wave filter devices that use other types of acoustic waves, such as boundary acoustic waves, are available. In a boundary acoustic wave filter, IDTs are provided at a boundary of lamination of a piezoelectric substrate and a dielectric layer to utilize boundary acoustic waves that propagate at the boundary. A boundary acoustic wave filter can be implemented by arranging electrodes substantially in the same manner as in a surface acoustic wave filter. Also, in boundary acoustic wave filter devices, it is also desirable to have both a balanced-to-unbalanced conversion function and a function of converting impedance by a large ratio.